This invention relates to the handling of webs and especially damp or wet paper webs and more particularly to a web handling device using air currents which suitably support the web while employing significantly reduced energy required to produce the air currents.
With the Production or processing of a web of a sensitive material, as, for example, paper or the like, it has proved advantageous for avoiding damage to the web or its surface to keep the web suspended for the duration of the drying process of the web material or its surface coating. Usually arrangements in connection with which air introduced from a blower over jets from one or both sides of the web is blown out against the web are used so that the web is borne by the air cushion so created. The air consequently circulating around the web accelerates the drying process, mostly assisted in addition by heating facilities extending over the width of the web. Since the trend in the production or processing of paper webs is toward ever higher operating speeds, while the drying rate of a unit area of web nonetheless has an upper limit, for example, owing to the maximum ability of the web material to take heat stress, this leads to the unit area having to be in a suspended state for a minimum period of time, and the webs must be guided under suspension for ever greater distances. Since, due to the relatively low strength, especially of wet paper webs, the maximally attainable distance between two successive suspension guideways is restricted, this development goes hand in hand with an ever greater number of support points necessary for the guidance of the web. On the one hand, this elevates the amount of air and therewith the energy required for guiding the web. On the other, it increases the space required by a drying device. The increased space requirement is usually taken into consideration by deflecting the paper web several times through which the floor area of the device is reduced with an equal length of freely suspended web section.
Devices with the aid of which a travelling web can be maintained or deflected in a suspended state are familiar in many forms of construction. Usually the devices include so-called nozzle boxes from which air exits in the direction of the web. Often the nozzles have the form of a slit (as for example in connection with a device in accordance with DE-PS 31 30 450). The nozzle boxes are arranged in the direction of travel of the web at a distance from one another, whereby the interstices serve as discharge pathways for the air, which then, as for example in the device familiar from GB-PS 13 07 695, basically escapes over the areas of the two web edges. The free escaping of the air over the web edges leads on the one hand to a great consumption of compressed air and therewith to a high energy demand, and on the other hand to an unstable running and unsatisfactory guidance of the web.
The device in accordance with DE-OS 29 32 794 seeks to improve on this by including air discharge nozzles running at right angles to the direction of travel of the web at the beginning and at the end of its deflection area, whereby the air current directed through them against the web induces an improved web support on the boundaries of the device and consequently prevents the web from touching the device. On the other hand, slit-like air discharge nozzles are provided in the area of the two web edges which inhibit the air discharge currents on the web edge side and consequently generate pressure heads in the web edge areas which exert a positive influence on the guidance of the web. The web deflection device described in DE-OS 29 32 794 has, however, the disadvantage that the energy demand is further increased owing to the auxiliary jets though which additional compressed air must be blown in.
The device according to DE-PS 31 04 656 represents a further development of DE-OS 29 32 794 in connection with which good characteristics with respect to the guidance and the stable running of the web are supposed to be obtained through a special selection of nozzle cross sections and their geometrical arrangement. The energy consumption to be expected is nevertheless still quite high due to the multiplicity of nozzles to be charged with compressed air.
A device for drying and/or keeping a moving web is familiar from DE 33 31 856 A1 which presents a series of air beams running parallel to one another, in which at all times a pair of elongated Coanda nozzles extending lengthwise toward the air beams are placed. Compressed air flows out of an air chamber provided in the device through the Coanda nozzles in the form of currents which provide contact-free support of the web of material guided over the device. In order to prevent an excessive amount of air from exiting on the sides of the device or on the side edges of the web of material, the first air barriers, which impede lateral air flow, are on the one hand provided at the ends of the air ducts formed between two adjacent air beams. On the other hand, the second air barriers, which border the web laterally are provided mounted on the upper side of the device and extending vertically upward this. The disadvantage in therefrom connection is that, for an effective reduction of air demand and therewith of energy consumption, the distance between the second air barriers and the web of material must be as small as possible, which is, however, possible only with a very exact guidance of the material web, as the web otherwise can be damaged by possible contacts with the air barriers.
Furthermore, devices for contact-free guidance of webs of material are familiar from GB 21 46 303 and DE 27 52 572 C2, in connection with which the special geometrical arrangement and construction of the air discharge nozzles lead to a stabilization of the travel of the web with the former. With the latter, these should make their applicability in connection with especially sensitive webs of material possible. Measures which could serve to reduce the high air requirement to be expected with both devices cannot be extracted from these writings.
A further formula for a web guidance device is familiar from DE 36 26 016 A1. With this formula, an improved guidance action on the web is achieved by providing air ducts installed at right angles to the direction of travel of the web as nozzle boxes which include lateral duct partitions forming a sharp angle to one another, whereby a series of nozzles is incorporated into each duct partition in such a manner that each nozzle lies opposite a deflector for the outflowing air formed by the other duct partition. By using the Coanda effect, a suitable air current is created for the stabilization of the web. The disadvantage with this web guidance arrangement is that the web has a tendency to arch along an isobar into the air duct, since reduced pressure prevails above each air duct on the basis of Bernoulli""s law for fluid media.
This tendency is counteracted by a further familiar device available from the Krieger company, Mxc3x6nchengladbach by providing auxiliary nozzles charged with compressed air on the floor of each air duct which subject the air duct to just such an additional amount of air that the isobar along which the web curves basically extends evenly over the surface of the device.
A very stable web guidance is, to be sure, obtained with this device, but the energy requirement necessitated for conveying the air, which comes to approximately 25 kW/m of web width, is quite high.
A device is known from FR-A 2 334 599 in which inlet apertures are provided between two adjacent slit-like air discharge nozzles through which additional air for supporting the web of material is drawn in by means of ejector action. The web guidance behavior should be improved by this measure.
Underlying the invention is the task of carrying on development of a device suitable for its type in such a way that the energy required is considerably reduced without having to accept sacrifices with regard to the web guidance behavior of the device.
Improving a device suitable for its type with respect to its functionality is the objective underlying the invention.
This task is accomplished by the invention for suspension guidance of a travelling web by means of air or another fluid medium, especially for use to deflect a wet paper web, with one or more air ducts (13) provided on at least one side of the web and which extend over the width of the web, with partition surfaces (14) provided alongside the air ducts (13), and with at least one nozzle arrangement (22, 22xe2x80x2) provided alongside each air duct (13), which communicates with a pressure chamber (10), which is acted upon by the fluid medium under an overpressure, wherein, in at least one air duct (13), inlet apertures (26) are provided for the fluid medium, which are fluidly connected with a second chamber independent of said pressure chamber (10), a pressure in said second chamber being lower than a pressure in said pressure chamber (10).
At least one part of the fluid medium drawn in by ejector action is drawn off from an area of increased pressure through the inlet apertures situated near the long end of the partition surfaces facing the web of material according to the invention, through which the efficiency and the web guidance behavior of the device is improved.
An additional amount of fluid medium can be conveyed into the air ducts by the ejector action of the medium flowing in under an overpressure owing to the nozzle arrangement provided in the air duct without an increased pump output being necessary for this through the inlet apertures provided in the air duct which are not fluidly connected with the pressure chamber. Compensating for the inrush of pressure above the air ducts now no longer requires actively blowing in additional amounts of fluid medium over separate injection nozzles installed in the air duct, but rather takes place automatically by means of fluid medium drawn in through the ejector action over the inlet apertures. Experiments have shown that the energy requirement of a device of appropriate type can be reduced by 25% through construction in accordance with the invention.
With the preferred embodiment, the inlet apertures each communicate with a separate intake chamber which has an inlet aperture for the fluid medium, through which the fluid medium is sucked off in a targeted manner from a predeterminable region of the environment and can, if necessary, be purified by a filter element or the like with a central location or installed in the intake chamber prior to passing through the inlet apertures.
To increase the degree of efficiency of the device, it is advantageous to provide inlet apertures near the edge of the band edge, as fluid medium is sucked out of an area of elevated pressure through this construction.
The elevated pressure of the fluid medium in the vicinity of the band edges and the flow components thereby directed outwardly can be additionally utilized by providing at least one flow glide sheet partially surrounding the inlet apertures.
Experiments have shown that energy savings of up to 25% can be obtained with a device of the invention constructed in this manner.
There exist numerous possibilities for construction of the air ducts in connection with the device of the invention. Experiments have shown that a configuration of the air duct is particularly advantageous where duct partitions are provided arranged on apposite sides of a transverse plane lying lengthwise in relation to the air duct which stands vertically on the tangential plane constructed on its line of intersection formed with the web.
In an especially convenient embodiment, the nozzle arrangement consists of a series of individual outlet apertures which are mounted in the one duct partition opposite the air current deflectors formed by the other duct partition. Very favorable air current conditions can be obtained through the fact that in a form of construction of this type, the duct partitions are constructed in the area of the emerging air currents as current deflectors. In particular, it is possible to create air currents especially low in turbulence which follow the surface of the respective duct partition according to the so-called Coanda effect.
A further evening out of the air currents can be created in that the discharge apertures are constructed as holes which are surrounded concentric deflector edges projecting into the air duct.
For creating a current extending symmetrically from the air duct over the surface of the device, it is advantageous if two duct partitions arranged immediately opposite each other, provided with a discharge outlet, form an air duct, and each of these duct partitions forms, at least in part, a deflector for currents of the fluid medium for fluid currents emerging from the outlets of the other duct partition. The tendency to form strong turbulence is diminished if the outflows of the one duct partition are mounted with lateral displacement in relation to the discharge apertures of the other duct partition.
If the device includes several air ducts, it is furthermore of particular advantage for diminishing the tendency to form harmful turbulence if the discharge apertures in one duct partition of an air duct are placed opposite the discharge apertures provided in the other duct partition of an adjacent air duct.
The device of the invention is then likewise especially suited for attaining low turbulence air currents when it is constructed so that at least one lateral area of each duct wall has a convex curvature in the direction of web movement and wherein the curved lateral area merges into one side of the partition surfaces since an even, edge free guidance of the air currents from the air ducts is thereby made possible.
The air ducts have partitions lying opposite one another forming an acute angle and being symmetrical about a transverse plane, the air duct having a lower boundary which coincides with the vertex of said acute angle is especially suited for using the ejector action of the inflowing medium. To obtain the maximal ejector effect, it is then advantageous to provide holes interrupting the lower boundary line of the air duct as inlet apertures.
It is however also possible with another preferred embodiment to provide inlet air nozzles over a considerable distance of the air duct extending along the lower boundary line instead of individual inlet apertures. This configuration of the inlet apertures is in particular recommended when both canal partitions consist of separate structural members which are then to be so arranged that they are placed at a distance in the lower region of the air duct corresponding to the necessary inlet air nozzle width. This form of construction has the advantage that no further processing steps are necessary for creating the inlet aperture.
It is beneficial in connection with a device designed for deflecting a web to provide several air ducts, the transverse planes of which intersect on a common line, so that the contour of the device is curved according to the direction of deflection.
For optimizing the guidance properties of the device, a configuration is here of advantage in connection with which the angular distance of neighboring transverse planes is almost constant. The formation of additional turbulence can be counteracted by the fact that the partition surfaces of the device are curved, advantageously with constant curvature radii.
For additional stabilization of the travelling web, it is advantageous if stabilization nozzles directed approximately radially outward are provided in the partition parts of the device placed at the beginning and end of the deflection area which communicate with the pressure chamber. Such stabilization nozzles help avoid harmful contacts between the web and the device even with a slight flutter of the web.
The device can also be used for suspension guidance or deflection of narrow webs without a major part of the flowing medium acted upon by pressure flowing unused into the surroundings with the disadvantages caused thereby owing to the pressure chamber being sectioned in a direction transverse to the direction of travel and owing to means for a separate pressure control being provided in the individual sections of the pressure chamber. Moreover, it is possible through this configuration of the device of the invention to raise the pressure of the sections acting on the edges of the web with the fluid medium relative to the further sections of the pressure chamber, through which the lateral guidance characteristics of the device are again improved. Advantageously, means for separate control of pressure consist of plate valves subject to continuous control interposed in individual sections.